Over a decade ago the IBM.TM. PC AT set the standard for the burgeoning personal computer (PC) industry. Today, IBM.TM. compatible architectures make up approximately 90% of the PC marketplace. Along side the tremendous growth of the PC market itself there has also grown up a very large market for add-on peripheral devices of all types. Because of the dominance of the IBM PC AT architecture, its peripheral bus, the so-called "AT" or "ISA" (Industry Standards Association) bus has become the dominant standard for connecting add-on peripheral devices to a PC system. The operation of ISA bus type computer systems is described in more detail in ISA System Architecture (New Revised Edition) by Tom Shanley and Don Anderson (.COPYRGT.1991 and 1993, Mindshare Press, 2202 Buttercup Drive, Richardson Tex. 75802, ISBN 1-881609-05-7) and incorporated herein by reference.
After more than ten years in use, however, the limitations of the ISA bus have caused a growing movement to define newer, more advanced peripheral buses. Two of the largest drawbacks to the ISA bus are its low level of performance, by today's standards, and that it may be very difficult to configure peripheral devices on an ISA bus as they give no explicit indication of their presence in the system or of their support requirements.
At present, the recently defined advanced peripheral bus, the so-called "PCI" bus is gaining momentum and looks likely to become the standard peripheral bus for a next generation of PCs. The operation of PCI bus type computer systems is described in more detail in PCI System Architecture (New Expanded Edition Rev 2.0 Compliant) by Tom Shanley (.COPYRGT.1993, Mindshare Press, 2202 Buttercup Drive, Richardson Tex. 75802, ISBN 1-881609-08-1) and incorporated herein by reference. However, new bus designs, such as the PCI bus, may have substantial obstacles and problems to overcome. Although newer bus type designs have technological advantages over the ISA bus design, there exists a large installed base of ISA bus type computers and compatible software. For the purposes of this application, these ISA bus type computers will be referred to as "legacy" computers or systems.
In order to provide a computer system which has the advantages of a new (e.g., PCI) bus design, it may be desirable to provide a computer with both PCI and ISA compatible buses. FIG. 1 illustrates an example of such a system. As shown in FIG. 1, a computer (e.g., PC, laptop, notebook or the like) may be provided with a CPU 100, a CPU interface to PCI Host controller 110 (hereinafter "PCI host controller") and a PCI bus 120. A number of peripheral devices 130,140 compatible with PCI bus architecture may be coupled to PCI bus 120. Such devices may include I/O controllers, memory, video controllers, hard drive controllers, or the like. A number of such devices may be provided coupled to PCI bus 120, however, for the purposes of illustration only two such devices 130,140 are shown here.
In addition to devices 130,140, a Primary PCI to ISA controller 150 (hereinafter "ISA controller") may be coupled to PCI bus 120. ISA controller 150 is itself a PCI bus compatible device and may be read from or written to through PCI bus 120 in a manner similar to devices 130,140. ISA controller 150 is coupled to ISA bus 160. ISA bus compatible devices 170,180 may be coupled to ISA bus 160. ISA bus compatible devices 170,180 may comprise any one of a number of peripheral devices compatible with the ISA bus architecture.
The dual bus system of FIG. 1 provides both PCI and ISA bus compatibility for both hardware and software. As the amount of Legacy hardware and software in existence is large, it is preferable to provide such a dual-bus system which is backwardly compatible.
However, the system of FIG. 1 may present some difficult hardware and software design problems. For example, the fundamental bus architecture of the ISA bus and PCI bus are different. The PCI bus is a synchronous bus, using an internal PCI bus clock to regulate the transfer of data and bus cycles. The ISA bus is an asynchronous bus architecture. Moreover, an ISA bus type computer system must be provided with direct memory access (DMA) channels and ISA type interrupt levels (IRQ) not present in the PCI type bus system. ISA controller 150 may be provided to convert ISA interrupt requests and generic ISA bus cycles into PCI compatible bus cycles, however there is no mechanism for handling DMA cycles.
While the use of such a controller may be suitable for a system having a single ISA bus, additional problems may be created if multiple ISA bus controllers are used. For example, for a portable computer, it may be desirable to provide a so-called docking station with a PCI bus interface. The docking station may be provided with a PCI to ISA bus interface controller to drive ISA bus compatible peripheral devices. If the portable computer also has an ISA to PCI bus interface controller, two ISA type buses may be present in one computer system at the same time. Since, as noted above, peripherals on an ISA system do not provide explicit indication of their presence on the ISA bus, it may be difficult to address ISA devices for read and write operations.